Automatic frequency controlling systems



April 12, 1960 N. N. P. SMITH ET AL 2,932,793

AUTOMATIC FREQUENCY CONTROLLING SYSTEMS Filed Sept. 17, 1957 LPL/mamSMU/th' rm'Hl/Mq HM 75141 Bald-unl? Ull ATTQRNEYS `AU'roivrA'rrcFREQUENCY coNrRoLLlNG SYSTEMS Norman -Neville Parker Smith, Chelmsford,and John Henry Howe, Ho'ckley, England, assignors to Marconis WirelessTelegraph Company Limited, London, England, a company of Great BritainApplication September 17, 19'57, Serial No. 684,566

Claims priority, application Great Britain November 20, 1956 3 Claims.(Cl. 328-134) This invention relates to automatic frequency controlsystems and more specifically to automatic frequency control circuitarrangements of the kind adapted to produce a control signal forcontrolling an oscillator in dependence upon the degreeY of lack ofsynchronism between oscillations generated -thereby andreferencefrequency oscillations. Control circuit arrangements of thekind referred to are frequently required to have a rapid pull in timeover a wide frequency range and a slow rate of follow oncenear-synchronism (i.e. synchronism near enough to lock the oscillator tothe reference frequency) has been establishd. A common example in whichthis requirement arises is that of a -television synchronizing pulsegenerator, and it is common in such cases to provide the control circuitarrangement with a switch adapted to change the values of theresistancecapacity time constant circuit elements in the control circuitso that in one position of the switch a fast control action is obtainedand in the other (when near-synchronism is establishd) a slow controlaction is obtained.,A

Sometimes the switch ismanually operatedbut sometimes it is a relayswitch whose operating current is derived from a circuit adapted todetect theV beat note which occurswhen the oscillator is not locked tothe reference frequency. 1

The invention seeks to provide improved circuit arrangements of the kindreferred to which shall provide,

the two dilerent rates of control action required purely; automaticallywithout the use of a switch or relay andv which shall, at the same time,and easy toset up.

According to this invention a control ,circuit arrangement adapted toproduce a control signal for controlling be simple, economical a sourceof oscillations in dependence upon the degree;

of lack of synchronism between the oscillations produced by said sourceand oscillations of reference frequency comprises means lactuated at thesource oscillation frequency for gating the reference oscillations toproduce periodic pulse signals of magnitude dependent upon theinstantaneous amplitudes of the reference oscillations at the moments ofgating, and means for applying said periodic pulse signals to a circuitVincluding a storage condenser in series with a resistance shuntedbyapair of'oppositely poled unilaterally conductive devices; "saiddevices being so biased as to be non-conductive when the appliedperiodic pulse signals correspond to' nearsynchronism between the sourcefrequency and the referencev frequency.

In a preferred circuit arrangement pulses of opposite polarity and ofsource frequency are applied to opposite ends of one diagonal of arectifier bridge the other diagonal of which is in series with a supplyof reference frequency, and the series circuit including said otherdiagonal and the reference frequency supply is connected across a secondseries circuit comprising a condenser in series with a resistanceshunted by two oppositely A, 2,932,793' Patented Apr. 12, 1960 ICC poledbiased rectiiers, said second series circuit being in turn in parallelwith a resistance-capacity network.

The invention is illustrated in and further explained in connectionwiththe accompanying drawings in which Figs. l and 2 are explanatorydiagrams showing known arrangements of the switch or relay type, thesefigures being provided for purposes of explanation, andFigs. 3 and 4 arecircuit diagrams of two embodiments of the v present invention. Likereferences are used for like parts In Fig. l the reference frequencythroughout the figures.

Referring to the drawings, the invention will be particularly describedwith reference to one of its most important applications,wnamely to atelevision synchronizing generator. In common television practice, inorder to establish the number of scanning lines per television field,the output from an oscillator running at twice the line repetitionfrequency is divided by a frequency divider, such as a conventionalcounting chain, the output from which is employed to determine the fieldscanning repetition rate. e It is common practice to synchronize thisfield frequency with that of a mains alternating current supply by meansof a discriminator or control circuit arrangement which compares the thereference frequency and produces a control Voltage which is thenemployed to control theoscillator of twice the line frequency. Figs. land 2 show two typical known discriminators or control circuitarrangements which are switch-controllable to provide either of two timeconstants, one giving slow action and being appropriate toYnear-synchronism conditions (ie. when the oscillator is flocked) and theother giving fast actio and being appropriate to pull in conditions.

Referring to Fig. l Aa reference frequency mains supply` is applied atterminals A, B to one end E of one diagonal of arectiiier bridge shownas consisting of four diodes. is injectedtby means of a transformer, thesecondary of which is in series with one bridge diagonal. Coincident eldfrequency gating pulses obtainedlin` any convenient way from theoscillation source (notgshown) to be controlled and of oppositepolarities are applied at C and D to opposite ends of the other diagonalof the bridge to gate the said bridge. :A condenser C1' is connectedacross the series circuit consisting of the transformer secondary andthe bridge and output is takenat F through resistances R1, R2, one of`which` is shunted by a switch S. The terminal F is connected to-theother side of condenser C1 through Va two-branch circuit, one of whichconsists of a resistance, R3 in series with a condenser C2 and the otherof which; consists of the condenser C3. The action of this arrangementis well known. The condenser C1 is charged to the potential existing atthe point E of Athe bridge at the instant when said bridge is renderedconducting by the gating pulses applied at C and D. The elements R1, R2,R3, C2 and rC3 constitute a low pass filter restricting the rate atwhich the output controlyoltage at` F can follow'the voltagefluctuations across V.condenser C1. If the switch S is closed `theresistance R1 is effectively cut out constant `of the networkaccordingly reduced. fThus,` by

operatingthe switch `Seither ofrtwo modes of operation fast or slow, isobtainable.

The known arrangement of Fig. 2 is similar to that of Fig. l except forthe omission of the condenser C1. Since this condenser C1 is omitted thepotential at point E is transferred directly to the condensers C2 andC3. Since, however, the bridge is conducting for only a small part ofeach cycle and the time constant of the low pass filter is long comparedwith the duration of the gating pulses, the apparent values of R1 and R2are increased inversely with the ratio of the pulse width to theinterval between pulses. Accordingly a very long effective time`generated field frequency with 4of circuit and the time.,

l onstantcan Abe .obtained without having to use very leading to onepairI of diagonal terminal connections of the bridge including the-highresistance'path leading to the said terminal connections. Another pairof diagonal terminal connections is provided for the other diagonal forthevbridge, the last mentioned connections being arranged in series withthe source of oscillations of reference frequency. As will be apparentthe switch S and the -resistance R2 of Figs. l and 2 are dispensed withandthe condenser C1'is now in series with aresistance R4 across whichrare two oppositcly vpoled rectitiers V1, V2 suitably biased by biaspotential`indicated by +V and `V. The amplitude ofthe mains frequencywave appearing at point E is made several times the amplitude required`to maintain synchronism once lock or near-synchronism h as beenestablished, so that, for any permitted variation of mains supplyfrequency, the-bridge will always open when the point E is relativelynear earth potential and similarly the potential values -l-V` and -Varetsochosen that the diodes 'V1 :.the beat noteproduced' in theunlockedcondition,A for with such a-known arrangement, synchronism maybetemporarily lost in the event of a sudden reference Vfrequencyiluctuation such as a change in phase. f

Fig. 4 is largely selfl explanatory and requires littlefurther'description. As will be` seen the ditferencebetween Figs. 3 and4 is rthat in the latter iigure the bias potentials-for the diodes V1and V2 are obtained without having to use a negative potential supplylWe claim:

y1. A control circuit arrangement adapated'to produce a control signalfor controlling a source of oscillations in dependence upon the degreeof lack of synchronism between the oscillations produced by said-,sourceand oscillations of reference frequency, said control circuitarrangement comprising a gating circuit, means to apply referenceoscillations to vsaid gating circuit, means for applied toA condenser C1will `also belimited in value. The bias and V2 are in the non-conductingcondition in these r circumstances and accordingly the charging currentintoV condenser C1 will be lrestricted by the resistance R4. Thisresistance isV large -so that the condenser C1 Vis rendered ineffectiveand the value of resistance R1 is electively increased beyond its actualphysicalV value as in the case ofthe knownv circuit of Fig. 2. Undersynchronous conditions, therefore, the circuit will operate with a longtime constant. If, however, Vthe near-synchronous condition does notyexist` and the oscillation source which is to be controlled in frequencyby the potential delivered at point "F is unlocked, the voltage existingat the point E when the bridge opens may be anything up (to vthe peakpotential of the reference frequency sine wave. This potential isapplied to condenser C1. lSince the diodes V1 and V2 are non-conductingonly fora `limited range of charging potentials, any applied voltagesAin the region of the peak potential at point E will render themconducting and re'sistance"R.1 will no longer llirriit the chargingcurrent. Providin'gf'that the time constant of the charging circuit is`low, condenser C1 will be fullyv charged during the pulse andtheeiective value of resistance R1 will be reduced to lits actual physicalvalue. Thus, under synchronous conditions,jtl;ie`

circuit operates with a low time constant ythus ensuring a rapid lpullin. n

An incidental but important advantage of the arrangement of Fig. 3 ascompared to known arrangements is that sudden fluctuations vin referencefrequency, e.g. changes in phase, will not change the effective timeconstant of the arrangement to the fast mode unless the change ofpotential at point E is suicient to cause they diodes V1, V2 to conduct.If, however, the Ailuctuaton is suiciently large the bias potential onthe diodes across resistance R4 will be overcome and the circuit willchange over to the fast mode. This constitutes a substantial practicaladvantage over a known 'circuit with a manual switch or with a relayswitch operated by detection of openingV said gating circuit at thefrequency* of said source whereby periodic pulse signals are producedhaving an amplitude dependent upon the instantaneous ampli tudeof thereference oscillations at the instant of gating,

a; seriesA circuit including a storage condenser and a.V

resistance in series, two oppositely poled unilaterallyeonductive`devices,ga,shunting circuit connecting said resistance andsaid unilaterally conductive devices in parallelfsaid storage condenserbeing connected toV said.

gating`v circuit so as to be charged therefrom, bias Ameans leading'fromsaid unilaterally conductive devicesadapt'ed `to render said devicesnon-conducting when the applied periodic pulse signals correspond tonear-synchroni'sm between the source frequency and the referencefrequency, and output means for Vdelivering an output con trol voltageconnected to said storage condenser.

2. 'A control circuit arrangement as set'forth in claim l wherein saidgating circuit comprises a rectifier bridge having two pair of diagonalterminal connections and means for applying pulses of opposite polarityand of the source frequency to one pair of diagonal terminal connectionsof said bridge, means connecting the other pair of terminal connectionsof said bridge in 4series with saidroscillations of reference frequency,the said series circuit vwhich includes said storage condenser and saidresistance in seriesbeing connected directly across said Vlast mentionedmeans and a resistance capacity net! work interposed between said seriescircuit and said output means.

' 3. A television synchronizing generator arrangement as setforth inclaim 1, in which saidV gating circuit includes a rectifier "bridgewhich operates at video tre"-` quencies for charging said storagecondenser andwherelin a low pass filter is interposed between saidseries cir-Y cuit which includes said storage condenserY and aresistance, and said output means, said low pass filter including aresistance for restricting the rate at which the output` control voltagecan follow the voltage fluctuations across said storage condenser.

References Cited in the tile of this patent UNITED STATES PATENTS2,676,252 Hugenholtz V Apr. 20,` '1954

